1. Field of the Invention
This invention relates to a valve mechanism for an internal combustion engine capable of driving an intake valve and an exhaust valve of the internal combustion engine to open and close at different driving timings in response to the driving state of the engine.
2. Description of the Related Art
In recent years, a valve mechanism (hereinafter referred sometimes as variable valve mechanism) has been developed and placed into practical use wherein the operational characteristic (opening and closing timings, open period) of an intake valve and an exhaust valve (hereinafter referred to generally as engine valves or simply as valves) provided in a reciprocating type internal combustion engine (hereinafter referred to as engine) can be changed over to an optimum characteristic in response to a load state or a speed state of the engine.
As one of mechanisms for changing over the working characteristic in such a valve mechanism as described above, for example, a mechanism has been developed wherein a low-speed cam having a cam profile suitable for a low-speed driving state of an engine and a high-speed cam having a cam profile suitable for high-speed driving of the engine are used selectively in response to the state of rotation of the engine so that the engine valves are operated between on and off (for example, refer to Patent Document 1).
An example of the structure of a conventional valve mechanism is described below with reference to FIGS. 10 to 12. As shown in FIGS. 10 and 11, two intake valves 11 and 12 and two exhaust valves 21 and 22 are provided on a cylinder head 10 above cylinders of an engine for each of the cylinders, and a valve mechanism 30 is provided in order to drive the intake valves 11 and 12 and the exhaust valves 21 and 22.
The valve mechanism 30 is formed from an intake valve driving system for driving the intake valve 11 and 12 and an exhaust valve driving system for driving the exhaust valve 21 and 22. The intake valve driving system includes a camshaft 31, cams 31a to 31c fixed to the camshaft 31, a rocker shaft 32, and rocker arms 33 to 35 supported for rocking motion on the rocker shaft 32 and rotationally driven by the cams 31a to 31c, respectively. The exhaust valve driving system includes the camshaft 31 commonly used with the intake system, cams 31d and 31e fixed to the camshaft 31, a rocker shaft 36, and rocker arms 37 and 38 (not shown in FIG. 11) supported for rocking motion on the rocker shaft 36 and rotationally driven by the cams 31d and 31e, respectively.
A variable valve mechanism 40 having a connection changeover mechanism 41 is provided at a portion of the intake valve driving system of the valve mechanism 30. The variable valve mechanism 40 is described briefly below.
Adjustment screws 33a and 34a are provided at one end of the rocker arms 33 and 34 from among the rocker arms 33 to 35 for the intake valve driving system, respectively, and stem end portions of the intake valves 11 and 12 contact with the one end of the rocker arms 33 and 34 through the adjustment screws 33a and 34a, respectively. Consequently, the intake valve 11 is opened and closed in response to the rocking motion of the rocker arm 33 and the intake valve 12 is opened and closed in response to the rocking motion of the rocker arm 34.
Further, rollers 33b and 34b are provided at the other end of the rocker arms 33 and 34, respectively. The rollers 33b and 34b contact with the low-speed cams 31a and 31b formed in the low-speed cam profile corresponding to low-speed driving of the engine, and, if the rocker arms 33 and 34 are rotationally driven in response to the low-speed cams 31a and 31b, respectively, then the intake valves 11 and 12 are opened with a characteristic suitable for low-speed driving.
On the other hand, the rocker arm (second rocker arm) 35 can contact, at a contacting projection 35a formed at one end thereof, with the rocker arms 33 and 34, and contacts, at a roller 35b formed at the other end thereof, with the high-speed cam 31c formed in the high-speed cam profile corresponding to high-speed driving of the engine.
Further, as shown in FIGS. 12(a) and 12(b), a cylinder 50 having an opening 53 is formed at a location at which the one end of the rocker arm 35 on the rocker arms 33 and 34 side can contact, and a piston 51 is built in the cylinder 50.
Operating oil (here, lubricating oil is used also as the operating oil) is supplied into the cylinder 50 through an oil path (communicating path) 32b from the rocker shaft 32 side, and, if pressure oil is supplied into the cylinder 50, then the piston 51 moves upwardly to close the opening 53 as shown in FIG. 12(b). On the other hand, if the pressure oil in the cylinder 50 is released to the atmosphere, then the piston 51 is moved downwardly by the biasing force of the return spring 52 to open the opening 53 as shown in FIG. 12(a).
Then, the connection changeover mechanism 41 for changing over the connection state between the rocker arms 33 and 34 and the rocker arm 35 is formed from such a piston 51 in the cylinder 50 as described above and an oil pressure adjustment apparatus (not shown) for adjusting the oil pressure in the cylinder 50, and the variable valve mechanism 40 is formed from the connection changeover mechanism 41 and the intake valve driving system.
According to the configuration described above, if the pressure oil in the cylinder 50 is exhausted by the oil pressure adjustment apparatus, then a space is formed in the opening 53 of the cylinder 50 [refer to FIG. 12(a)]. In this instance, if the rocker arm 35 is rotationally driven by the high-speed cam 31c, then the contacting projection 35a advances into the space. However, the contacting projection 35a does not contact with the rocker arms 33 and 34 themselves, and the rocker arm 35 exhibits a so-called miss swing state (rocker arm non-contacting state). Accordingly, the rocker arms 33 and 34 are rotationally driven in response to the individually corresponding low-speed cams 31a and 31b, and the intake valves 11 and 12 are driven to open and close with the characteristic suitable for low-speed driving (low-speed driving mode).
On the other hand, if the oil pressure in the cylinder 50 is increased by the oil pressure adjustment apparatus, then the piston 51 is placed into a contacting state wherein it is projected, and the opening 53 of the cylinder 50 is closed with the piston 51 [refer to FIG. 12(b)]. Accordingly, upon rocking of the rocker arm 35, the contacting projection 35a at the one end of the rocker arm 35 contacts with a side face (contacting face) 54 of the piston 51 to rock the rocker arms 33 and 34 through the piston 51 (rocker arm contacting state). At this time, the rocker arms 33 and 34 are rotationally driven by the rocker arm 35 to rock in response to the high-speed cam 31c while moving away from the low-speed cams 31a and 31b thereby to open and close the intake valves 11 and 12 with the characteristic corresponding to high-speed driving of the engine (high-speed driving mode).
Patent Document 1: Japanese Patent Laid-Open No. 2003-343226